Pellets might be “only” an intermediate product, but their size, shape, and consistency matter in subsequent processing operations.
This becomes much more important when it comes to the ever-increasing demands positioned on compounders. No matter what equipment they now have, it never seems suited for the next challenge. An increasing number of products may require additional capacity. A fresh polymer or additive may be too tough, soft, or corrosive to the existing equipment. Or maybe the job needs a different pellet shape. In these cases, compounders need in-depth engineering know-how on processing, and close cooperation making use of their pelletizing equipment supplier.
The first step in meeting such challenges starts with equipment selection. The most prevalent classification of pelletizing processes involves two classes, differentiated by the state of the plastic material at the time it’s cut:
•Melt pelletizing (hot cut): Melt originating from a die that may be almost immediately cut into pvc pellet which can be conveyed and cooled by liquid or gas;
•Strand pelletizing (cold cut): Melt coming from a die head is transformed into strands that happen to be cut into pellets after cooling and solidification.
Variations of these basic processes can be tailored to the specific input material and product properties in sophisticated compound production. Within both cases, intermediate process steps and different levels of automation may be incorporated at any stage in the process.
For the greatest solution for your personal production requirements, start out with assessing the status quo, along with defining future needs. Develop a five-year projection of materials and required capacities. Short-term solutions often end up being higher priced and fewer satisfactory after a period of time. Though virtually every pelletizing line with a compounder need to process a number of products, virtually any system might be optimized simply for a tiny range of the entire product portfolio.
Consequently, all of the other products will need to be processed under compromise conditions.
The lot size, in combination with the nominal system capacity, will have a very strong influence on the pelletizing process and machinery selection. Since compounding production lots are usually rather small, the flexibleness of your equipment is usually a big issue. Factors include quick access for cleaning and service and the opportunity to simply and quickly move from a product to the next. Start-up and shutdown of the pelletizing system should involve minimum waste of material.
A line utilizing a simple water bath for strand cooling often is the first selection for compounding plants. However, the individual layout may differ significantly, as a result of demands of throughput, flexibility, and degree of system integration. In strand pelletizing, polymer strands exit the die head and are transported via a water bath and cooled. After the strands leave this type of water bath, the residual water is wiped from your surface by means of a suction air knife. The dried and solidified strands are transported on the pelletizer, being pulled into the cutting chamber by the feed section at a constant line speed. In the pelletizer, strands are cut from a rotor along with a bed knife into roughly cylindrical pellets. This can be put through post-treatment like classifying, additional cooling, and drying, plus conveying.
In the event the requirement is for continuous compounding, where fewer product changes come to mind and capacities are relatively high, automation can be advantageous for reducing costs while increasing quality. This kind of automatic strand pelletizing line may utilize a self-stranding variation of this sort of pelletizer. This is certainly characterized by a cooling water slide and perforated conveyor belt that replace the cooling trough and evaporation line and give automatic transportation in the pelletizer.
Some polymer compounds are quite fragile and break easily. Other compounds, or some of their ingredients, could be very understanding of moisture. For such materials, the belt-conveyor strand pelletizer is the best answer. A perforated conveyor belt takes the strands through the die and conveys them smoothly towards the cutter. Various options of cooling-water spray, misters, compressed-air Venturi dies, air fan, or combinations thereof-allow for a great deal of flexibility.
If the preferred pellet shape is a lot more spherical than cylindrical, the best alternative is surely an underwater hot-face cutter. Using a capacity vary from from about 20 lb/hr to a number of tons/hr, this system is relevant to all of materials with thermoplastic behavior. Functioning, the polymer melt is divided in a ring of strands that flow via an annular die into a cutting chamber flooded with process water. A rotating cutting head within the water stream cuts the polymer strands into rigid pvc compound, that are immediately conveyed out of your cutting chamber. The pellets are transported being a slurry towards the centrifugal dryer, where they can be separated from water from the impact of rotating paddles. The dry pellets are discharged and delivered for subsequent processing. Water is filtered, tempered, and recirculated straight back to the method.
The main parts of the program-cutting head with cutting chamber, die plate, and begin-up valve, all on a common supporting frame-are one major assembly. All of the other system components, including process-water circuit with bypass, cutting chamber discharge, sight glass, centrifugal dryer, belt filter, water pump, heat exchanger, and transport system could be selected coming from a comprehensive selection of accessories and combined in to a job-specific system.
In every single underwater pelletizing system, a fragile temperature equilibrium exists inside the cutting chamber and die plate. The die plate is both continuously cooled by the process water and heated by die-head heaters as well as the hot melt flow. Decreasing the energy loss through the die plate to the process water results in a considerably more stable processing condition and increased product quality. So that you can reduce this heat loss, the processor may pick a thermally insulating die plate and move to a fluid-heated die.
Many compounds are quite abrasive, leading to significant wear and tear on contact parts like the spinning blades and filter screens within the centrifugal dryer. Other compounds may be responsive to mechanical impact and generate excessive dust. For these two special materials, a fresh type of pellet dryer deposits the wet pellets over a perforated conveyor belt that travels across an aura knife, effectively suctioning off the water. Wear of machine parts as well as damage to the pellets may be cut down tremendously in contrast to an impact dryer. Considering the short residence time about the belt, some type of post-dewatering drying (for example using a fluidized bed) or additional cooling is normally required. Benefits of this new non-impact pellet-drying solution are:
•Lower production costs because of long lifetime of parts getting into contact with pellets.
•Gentle pellet handling, which ensures high product quality and much less dust generation.
•Reduced energy consumption because no additional energy supply is needed.
Various other pelletizing processes are rather unusual inside the compounding field. The simplest and cheapest means of reducing plastics to a appropriate size for even more processing might be a simple grinding operation. However, the resulting particle size and shape are exceedingly inconsistent. Some important product properties will even suffer negative influence: The bulk density will drastically decrease and also the free-flow properties of your bulk can be bad. That’s why such material are only acceptable for inferior applications and must be marketed at rather low priced.
Dicing have been a typical size-reduction process because the early 20th Century. The necessity of this process has steadily decreased for nearly thirty years and currently will make a negligible contribution to the current pellet markets.
Underwater strand pelletizing is really a sophisticated automatic process. But this procedure of production is commonly used primarily in a few virgin polymer production, like for polyesters, nylons, and styrenic polymers, and it has no common application in today’s compounding.
Air-cooled die-face pelletizing is really a process applicable only for non-sticky products, especially PVC. But this material is much more commonly compounded in batch mixers with air conditioning and discharged as dry-blends. Only negligible amounts of PVC compounds are transformed into pellets.
Water-ring pelletizing can also be an automatic operation. Yet it is also suitable only for less sticky materials and finds its main application in polyolefin recycling and also in some minor applications in compounding.
Choosing the right pelletizing process involves consideration in excess of pellet shape and throughput volume. As an example, pellet temperature and residual moisture are inversely proportional; which is, the better the product temperature, the less the residual moisture. Some compounds, including various types of TPE, are sticky, especially at elevated temperatures. This effect can be measured by counting the agglomerates-twins and multiples-in the bulk of pellets.
Within an underwater pelletizing system such agglomerates of sticky pellets can be generated in 2 ways. First, right after the cut, the surface temperature of the pellet is simply about 50° F over the process temperature of water, even though the core in the pellet is still molten, and also the average pellet temperature is merely 35° to 40° F below the melt temperature. If two pellets come into contact, they deform slightly, creating a contact surface between your pellets which may be clear of process water. In that contact zone, the solidified skin will remelt immediately on account of heat transported through the molten core, as well as the pellets will fuse to one another.
Second, after discharge from the clear pvc granule in the dryer, the pellets’ surface temperature increases as a result of heat transport through the core on the surface. If soft TPE pellets are held in a container, the pellets can deform, warm contact surfaces between individual pellets become larger, and adhesion increases, leading again to agglomerates. This phenomenon might be intensified with smaller pellet size-e.g., micro-pellets-ever since the ratio of surface area to volume increases with smaller diameter.
Pellet agglomeration can be reduced by adding some wax-like substance towards the process water or by powdering the pellet surfaces just after the pellet dryer.
Performing several pelletizing test runs at consistent throughput rate will give you a solid idea of the maximum practical pellet temperature for that material type and pellet size. Anything dexrpky05 that temperature will raise the volume of agglomerates, and anything below that temperature increases residual moisture.
In some cases, the pelletizing operation can be expendable. This is true only in applications where virgin polymers might be converted instantly to finished products-direct extrusion of PET sheet from the polymer reactor, as an example. If compounding of additives as well as other ingredients adds real value, however, direct conversion is not really possible. If pelletizing is essential, it usually is better to know your options.